Hi everyone!
Well, my HP-67 had, thanks to all of you, been running along quite happily, balancing my checkbook and teaching my daughter her multiplication tables.
Suddenly, it started acting flaky again. It acts as if the ACT chip is bad AGAIN, but not quite the same.
When first turned on, the calc shows the correct display, then quickly scrambles to 0's or off. On a whim, I applied 6 volts across the battery contacts, and it will run for a few seconds properly, then it starts to display strange things; then either blanking out completely or displaying all 0's.
Any ideas? Why would it work for a few seconds on power-up, then fail? Does the fact that it runs longer with a higher voltage give any clues? Did it burn out yet another ACT chip (remember-- it's one from an HP-25)? Bad power suddenly?
As always, I appreciate the support... it seems as if this HP-67 really belongs more to the world-wide HP community than to just me.
Unhappy programming today,
Michael
Hello;
despite the fact the ACT IC is burned out or not, I have a few junk in here (mostly old HP2x series). I'd be glad sending you any IC's you need that I have. Just let me know.
Cheers.
Two thoughts : Firstly, check/replace all the eletrolytic capacitors (including tantalum bead ones) on the logic PCB. This might be power-supply related.
You could also check that the internal power lines are correct (Vss is pin 1 of the ACT, Vgg is pin 2 of the ACT, measure wrt ground (-ve battery contact, pin 12 of the ACT). Vss should be around 6.2V, Vgg around -12V.
Also get a can of freezer spray. Run the machine with the back cover off (use a rubber band to hold the battery in place :-)) and when it acts up, then squirt all the chips with freezer to see if you can get it to work again. Maybe something is thermally sensitive.
Tony,
Well, I put in all new capacitors with no luck. I swapped another ACT chip from an HP-25, and the 67 works again. Oddly, so does the HP-25 it came from....
Maybe they're not completely compatible, or maybe the 67 is doing something to keep burning them out somehow?? Anyway, it's working again. As long as I can keep a supply of functioning ACT chips, it looks like I'll be O.K. <grin>
Michael